In order to enhance performance and utility in a computer system a technique called virtual memory is frequently used. One motivation for using virtual memory is to allow multiple programs to simultaneously share a computer system's main memory. This is achieved by allocating individual portions (referred to as blocks or segments) of the main memory to each of the programs being run (also referred to as a tasks). Virtual memory systems are also used in cases when a single program is too large to fit into main memory. In this case, portions of the program are stored in secondary memory and the virtual memory system assists in retrieving these portions from the secondary memory.
Virtual memory is implemented by using virtual addresses at the task or program level--each task having its own set of independent addresses. When a program performs a memory access, the virtual addresses are translated into physical addresses that may or may not be the same as other physical addresses for other tasks. The translation may be successful, leading to an access to main memory using that physical address. The translation may be unsuccessful, indicating that physical, or main memory has not been allocated for that virtual address, leading to a processor exception, from which the program may be aborted or physical memory may be allocated and the task restarted. To enhance the translation performance, virtual addresses are translated to physical addresses using information stored in a translation look-aside buffer (TLB), also known as a translation cache. The TLB provides the information that defines the mapping for each of the virtual addresses.
There are basically two categories of virtual memory systems presently utilized: paging and segmentation. Paging systems typically use fixed size blocks for allocating memory to processes. Segmentation, in contrast uses variable size blocks which may range from a value as small as one byte. Paging suffers from the disadvantage that sections of contiguous physical memory become unused because the page size is fixed: this is known as internal fragmentation. Segmentation, on the other hand, has disadvantages that the variable-sized segments may create unused regions of memory as segments are allocated, deallocated, and reallocated in arbitrary order, leaving holes in the consecutive memory allocation which become unused or unusable because they are not of a suitable size. A hybrid of the two categories has been employed in prior art systems in which segmentation and paging are both employed together.
Virtual memory systems may also employ a memory cache system to minimize virtual memory misses which includes a cache data storage and its corresponding cache tag storage. The cache stores recently accessed data and the tag storage stores a portion of the virtual address or physical address, providing the means by which it can be determined whether the cache contains the requested address. Only a portion of the address is usually required because the remaining portion of the address is used to locate (index) a reference within the cache data and tag storage, and so need not be checked again.
Caches may use either a virtual or physical address to index the cache, known as a virtual-index cache or a physical-index cache. Additionally, caches may use either a virtual or physical address stored and compared against in the cache tag storage, known as a virtual-tag cache or a physical-tag cache. Virtual-index and virtual-tag caches are generally able to attain higher peak performance, but add constraint to the mapping of addresses available when sharing data or changing the address mapping. In particular, the problem called aliasing occurs, in which two tasks use different virtual addresses to reference the same physical memory. Aliasing may require that tasks sharing memory space use identical or similar virtual addresses.
Since virtual memory allows two processes to share the same portion of physical memory such that each of the processes' virtual memory are mapped to different addresses, it is necessary to implement a protection scheme that prevents one task (i.e. a set of program instructions) from modifying a portion of memory, unless specifically allowed. Typically, tasks are assigned privilege levels which indicate the tasks its ability to modify areas within physical memory and establish a control hierarchy, where higher privileged tasks are able to manipulate the storage of lower privileged tasks, including the possibility of higher privileged tasks manipulating the state of the virtual memory system itself.
One implementation of a protection scheme presently employed by virtual memory systems are "gateways" or "call gates" that function to provide a given task limited access privilege to areas in the physical memory having higher privilege than the task. The disadvantages of this prior art gateway implementation is that they utilize the CPU's status register requiring additional instructions in order to modify the status registers, and fail to provide securely initialized machine state, requiring additional instructions to initialize CPU registers used to access privileged memory registions. As a result, prior art gateway methods tend to reduce overall system performance by increasing execution times.
The present invention is a virtual memory system that performs virtual address-to-physical address translations in a manner that increases the overall efficiency and flexibility of the virtual memory system.